The objective of the present research proposal is to define the changes in distribution which cortical projection neuron systems undergo during the early postnatal period and to identify factors which influence the ultimate distribution of these neurons. A number of different cortical projection neuron systems are discretely distributed in the parietal cortex of the adult rat. At least three of these projection systems, namely the callosal, the corticotectal and the corticospinal, are initially continually distributed and become restricted to discrete areas during the first two postnatal weeks. A major goal of the current proposal is to document the transition from continuous to discrete distribution of neurons in the corticospinal and corticotectal systems, as well as to examine other systems, such as the corticothalamic, corticopontine and corticotrigeminal, for similar changes. A second major goal is to determine if the normal distribution of cortical projection neurons can be altered during the early postnatal period. The effects of several different types of manipulations will be tested. First, the effect of sensory input manipulations (neonatal vibrissae damage) on the distribution of callosal projection neurons will be determined. The second major manipulation will be to remove muscle target tissue by amputation and determine if this has any effect on the distribution of corticospinal neurons. Finally, the direct effects of central target manipulation will be tested. We will seek to determine if removal of one target (severing the spinal cord, for example) influences the adult distribution of other projection systems (in this case, corticotectal or corticothalamic). The major techniques to be employed are the retrograde transport of horseradish peroxidase, autoradiography utilizing the anterograde transport of labeled amino acids, and double labeling with retrogradely transported dyes.